Detection of Potentially Explosive Methane Levels Using a Solid-State Infrared Source

Detection of methane gas which may be approaching the concentration limit when explosive ignition could occur is an important industrial problem. Optical methods for gas detection are attractive, and near-infrared (IR) wavelengths are especially suited to the detection of hydrocarbon gases. Unfortunately, temperature-related drift of solid-state IR sources is problematic. A method for stabilizing the response of a near-IR solid-state gas detection system operating at 2350 nm is presented in this paper. The system employs a broadband LED source and a wideband photodetector. Because IR absorption in the gas cell is used as an indirect measure of gas concentration, it is necessary to stabilize the optical source power. We approach this problem by employing a novel two-frequency pulsed excitation method. Stable measurements suitable for detecting the presence of methane gas at a concentration of 50% of the lower explosive limit are experimentally demonstrated. The response of the system is validated against the HITRAN IR spectroscopy database, by incorporating the emitter and detector IR profiles. Good agreement between the derived gas concentration and theoretical predictions based on standard gas absorption models is demonstrated for 2.5% methane in air, which is a critical point for determining the presence of potentially explosive mixtures.